The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons. The feedstock (e.g. natural gas, associated gas, coal-bed methane, heavy oil residues, peat, biomass, coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthetic gas or syngas). The synthesis gas is then fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
For a general overview for the Fischer-Tropsch process reference is made to Fischer-Tropsch Technology, Studies in Surface Science and Catalysis, Vol. 152, Steynberg and Dry (ed.) Elsevier, 2004, Amsterdam, 0-444-51354-X. Reference is further made to review articles in Kirk Othmer, Encyclopedia of Chem. Techn. and Ullmann's Encyclopedia of Ind. Chem., Vol. 6, 4th edition, p. 584 ff.
As mentioned above, “coal” and heavy oil residues are examples of feedstocks for the Fischer-Tropsch process. However, there are many other solid or very heavy (viscous) fossil fuels which may be used as feedstock for the process, including solid fuels such as anthracite, brown coal, bitumous coal, sub-bitumous coal, lignite, petroleum coke, peat and the like, and heavy residues (tar sand, residue from refineries such as residual oil fractions boiling above 360° C., especially above 550° C., more especially above 750° C.) directly derived from crude oil, or from oil conversion processes such as thermal cracking, catalyte cracking, hydrocracking. All such types of fuels have different levels of ‘quality’, that is the proportions of carbon and hydrogen, as well as substances regarded as ‘impurities’, generally sulfur and sulfur-based compounds.
Gasification of solid carbonaceous fuels such as coal is well known, and generally involves milling or otherwise grinding the fuel to a preferred size or size range, followed by heating the fuel with oxygen in a gasifier. This creates the mixture of hydrogen and carbon monoxide referred to as syngas. However, the proportion of carbon and hydrogen in solid carbonaceous fuels is generally such that the hydrogen/carbon monoxide (H2/CO) ratio in the syngas formed is generally less than 1, whereas Fischer-Tropsch processes based on cobalt-catalysts generally desire a H2/CO ratio in the syngas to the synthesis reactor from 1.9 to approximately 2.2, frequently 2.0-2.1. Higher ratio syngases are also desired for other parts or sections of a Fischer-Tropsch plant: some parts may desire a substantially pure hydrogen stream, that is, a very high H2/CO ratio.